In JP 2001-147278A, a rain sensor provided in a vehicle is disclosed which detects rain drops by including a camera 20, an image processor 130, a controller 140, and a memory 150 (shown in FIG. 9).
The camera 20 takes an image of a predetermined area in a windshield 10 of the vehicle. The image processor 130 obtains a binarized image by processing the image taken by the camera 20. The controller 140 stores the binarized image into the memory 150. Every time the image processor 130 obtains a binarized image of the predetermined area, the controller 140 determines an amount of rain W on the windshield 10 based on the currently binarized image and a previously binarized image stored in the memory 150. The controller 140 controls a wiper 70 through a motor 60 based on the determined amount.
An operation of the conventional rain sensor is described in detail below. An original image taken by the camera 20 is inputted into the image processor 130. FIG. 10A shows an example of the original image. The image processor 130 then executes noise reduction, edge extraction, and binarization of the original image to obtain a binarized image. The binarized image is inputted into the controller 140. FIG. 10B shows an example of the binarized image.
The controller 140 makes a determination whether external substances are on the windshield 10 based on the binarized image. If the determination is affirmative, the controller 140 reads a previously stored binarized image in the memory 150. The controller then makes a determination whether the external substances are rain drops by comparing the currently obtained binarized image with the previously stored binarized image. If the determination is affirmative, the controller 140 detects an amount of the rain drops. If the detected amount is larger than a predetermined threshold, the controller 140 continuously operates the wiper 70 using the motor 60. If the detected amount is smaller than the predetermined threshold, the controller 140 intermittently operates the wiper 70. The rain sensor thus reduces troublesomeness on a driver of the vehicle because the driver need not judge an amount of rain to operate the wiper 70.
The rain sensor described above detects the external substances on the windshield 10 by executing edge extraction of the original image. In heavy rain where the amount of the rain drops is huge, borders between the rain drops quickly disappear and, thereby, the entire windshield 10 is covered with the rain. In this case, it is difficult for the controller 140 to extract edges of the rain drops. In another case that a mud splash causes a large amount of mud to be attached to the windshield 10, it is also difficult for the controller 140 to extract edges of the mud. In this context, the conventional rain sensor has room for improvement.